The proof can be easily driven with a similar approach to that of theorem 1 in [1]}.
Therefore, the resultant optimization problem of maximizing the sum-rate is written as
\(\begin{array}{cl}\max \limits _{\mathbf {W}, \mathbf {v}}& R^{[sum]}_{\text{FDMA}}\\\operatorname{s.t.}&|\theta _i|=1, i=1,...,M,\\&\operatorname{trace}\left(\mathbf {W}\mathbf {W}^H\right)\le P.\end{array}\) 
